SENDMSG(3P)                                         POSIX Programmer's Manual                                        SENDMSG(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       sendmsg - send a message on a socket using a message structure

SYNOPSIS
       #include <sys/socket.h>

       ssize_t sendmsg(int socket, const struct msghdr *message, int flags);


DESCRIPTION
       The  sendmsg()  function  shall  send a message through a connection-mode or connectionless-mode socket. If the socket is
       connectionless-mode, the message shall be sent to the address specified by msghdr. If the socket is connection-mode,  the
       destination address in msghdr shall be ignored.

       The sendmsg() function takes the following arguments:

       socket Specifies the socket file descriptor.

       message
              Points  to  a  msghdr structure, containing both the destination address and the buffers for the outgoing message.
              The length and format of the address depend on the address family of the socket. The msg_flags member is ignored.

       flags  Specifies the type of message transmission. The application may specify 0 or the following flag:

       MSG_EOR
              Terminates a record (if supported by the protocol).

       MSG_OOB
              Sends out-of-band data on sockets that support out-of-bound data.  The significance and semantics  of  out-of-band
              data are protocol-specific.



       The msg_iov and msg_iovlen fields of message specify zero or more buffers containing the data to be sent.  msg_iov points
       to an array of iovec structures; msg_iovlen shall be set to the dimension of this array. In  each  iovec  structure,  the
       iov_base  field  specifies a storage area and the iov_len field gives its size in bytes. Some of these sizes can be zero.
       The data from each storage area indicated by msg_iov is sent in turn.

       Successful completion of a call to sendmsg() does not guarantee delivery of the message. A return value of  -1  indicates
       only locally-detected errors.

       If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does
       not have O_NONBLOCK set, the sendmsg() function shall block until space is available. If space is not  available  at  the
       sending  socket  to  hold  the  message  to  be  transmitted and the socket file descriptor does have O_NONBLOCK set, the
       sendmsg() function shall fail.

       If the socket protocol supports broadcast and the specified address is a  broadcast  address  for  the  socket  protocol,
       sendmsg() shall fail if the SO_BROADCAST option is not set for the socket.

       The socket in use may require the process to have appropriate privileges to use the sendmsg() function.

RETURN VALUE
       Upon  successful  completion,  sendmsg() shall return the number of bytes sent. Otherwise, -1 shall be returned and errno
       set to indicate the error.

ERRORS
       The sendmsg() function shall fail if:

       EAGAIN or EWOULDBLOCK
              The socket's file descriptor is marked O_NONBLOCK and the requested operation would block.

       EAFNOSUPPORT
              Addresses in the specified address family cannot be used with this socket.

       EBADF  The socket argument is not a valid file descriptor.

       ECONNRESET
              A connection was forcibly closed by a peer.

       EINTR  A signal interrupted sendmsg() before any data was transmitted.

       EINVAL The sum of the iov_len values overflows an ssize_t.

       EMSGSIZE
              The message is too large to be sent all at once (as the socket requires), or the msg_iovlen member of  the  msghdr
              structure pointed to by message is less than or equal to 0 or is greater than {IOV_MAX}.

       ENOTCONN
              The socket is connection-mode but is not connected.

       ENOTSOCK
              The socket argument does not refer to a socket.

       EOPNOTSUPP
              The socket argument is associated with a socket that does not support one or more of the values set in flags.

       EPIPE  The  socket  is  shut down for writing, or the socket is connection-mode and is no longer connected. In the latter
              case, and if the socket is of type SOCK_STREAM, the SIGPIPE signal is generated to the calling thread.


       If the address family of the socket is AF_UNIX, then sendmsg() shall fail if:

       EIO    An I/O error occurred while reading from or writing to the file system.

       ELOOP  A loop exists in symbolic links encountered during resolution of the pathname in the socket address.

       ENAMETOOLONG
              A component of a pathname exceeded {NAME_MAX} characters, or an entire pathname exceeded {PATH_MAX} characters.

       ENOENT A component of the pathname does not name an existing file or the path name is an empty string.

       ENOTDIR
              A component of the path prefix of the pathname in the socket address is not a directory.


       The sendmsg() function may fail if:

       EACCES Search permission is denied for a component of the path prefix; or write access to the named socket is denied.

       EDESTADDRREQ
              The socket is not connection-mode and does not have its peer address set, and no destination  address  was  speci-
              fied.

       EHOSTUNREACH
              The destination host cannot be reached (probably because the host is down or a remote router cannot reach it).

       EIO    An I/O error occurred while reading from or writing to the file system.

       EISCONN
              A destination address was specified and the socket is already connected.

       ENETDOWN
              The local network interface used to reach the destination is down.

       ENETUNREACH
              No route to the network is present.

       ENOBUFS
              Insufficient resources were available in the system to perform the operation.

       ENOMEM Insufficient memory was available to fulfill the request.


       If the address family of the socket is AF_UNIX, then sendmsg() may fail if:

       ELOOP  More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the pathname in the socket address.

       ENAMETOOLONG
              Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.


       The following sections are informative.

EXAMPLES
       Done.

APPLICATION USAGE
       The select() and poll() functions can be used to determine when it is possible to send more data.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       getsockopt(),  poll(), recv(), recvfrom(), recvmsg(), select(), send(), sendto(), setsockopt(), shutdown(), socket(), the
       Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003  Edition,  Standard  for
       Information  Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
       right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
       discrepancy  between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
       Standard  is  the  referee   document.   The   original   Standard   can   be   obtained   online   at   http://www.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                   SENDMSG(3P)